384 Darwin, and after Darwin. 
males rival one another to the best of their musical 
ability. 
Thus there can be no question that the courtship 
of birds is a highly elaborate business, in which the 
males do their best to surpass one another in charming 
the females. Obviously the inference is that the maled 
do not take all this trouble for nothing ; but that the \ 
females give their consent to pair with the males \ 
whose personal appearance, or whose voice, proves yy 
be the most attractive. But, if so, the young of the 
male bird who is thus se/ected will inherit his superior 
beauty; and thus, in successive generations, a con- 
tinuous advance will be made in the beauty of 
plumage or of song, as the case may be,—both the 
origin and development of beauty in the animal world 
being thus supposed due to the esthetic taste of 
animals themselves. 
Such is the theory of sexual selection in its main 
outlines; and with regard to it we must begin by 
noting two things which are of most importance. In 
the first place, it is a theory wholly and completely 
distinct from the theory of natural selection ; so that 
any truth or error in the one does not in the least 
affect the other. The second point is, that there is 
not so great a wealth of evidence in favour of sexual 
selection as there is in favour of natural selection; 
and, therefore, that while all naturalists nowadays 
accept natural selection as a (whether or not ¢/e) cause 
of adaptive, useful, or life-preserving structures, there 
is no such universal—but only a very general—agree- 
ment with reference to sexual selection as a cause 
of decorative, beautiful, or life-embellishing struc- 
tures. Nevertheless, the evidence in favour of sexual 
